1. Field of the Invention
The present invention relates to stages that support and move a workpiece, and in particular relates to magnetic levitation (maglev) stages.
2. Description of the Related Art
There are many industrial applications that require precise movement and positioning of a workpiece. In the case of semiconductor manufacturing, the workpiece is a semiconductor wafer moved and positioned with respect to the projection lens (or other system reference) of the photolithography system. This movement and positioning is accomplished by placing the workpiece on a precision stage capable of moving in three linear and three rotational dimensions. The required movements in the linear dimensions are often as small as nanometers and in the rotational dimensions as small as micro-radians.
The first lithography systems used stages that employed direct drive mechanisms for providing motion, such as contact bearings and lead screws. Rails were used to provide sliding or rolling contact of the stage over a large stage base. As the demands on the lithography process increased (e.g., greater throughput, faster printing, more accurate alignment, smaller device features, etc.), demands on stage performance (e.g., greater stage speed, higher accuracy, less vibration, longer lifetime, etc.) also increased. This led to the development and use of more precise and stable non-contact bearings, such as gas and fluid bearings, and later, magnetic levitation (xe2x80x9cmaglevxe2x80x9d) bearings.
Maglev bearings have a number of advantages over other types of non-contact bearings, such as providing six-degrees-of-freedom (DOF) from a monolithic workpiece carrier, mechanical simplicity, and inherent isolation from seismic disturbance forces. U.S. Pat. Nos. 5,157,296, 5,196,745, 5,294,854 and 5,699,621 describe various types of maglev stages for lithography systems. Further, the article by M. E. Williams et al., entitled Magnetic Levitation Scanning Stages for Extreme Ultraviolet Lithography, ASPE 14th annual meeting, Monterey Calif., November 1999, also discusses maglev stages.
Prior art lithography system stages have an acceleration of about 0.15 g and velocities under 300 mm/sec. Also, the platens of prior art stages have not been especially massive, particularly when compared to the anticipated needs of next-generation stages. Consequently, vibration generation from stage movement has not been as severe a problem in the past However, because of the demands for next-generation lithography, the next-generation lithography systems will require more massive stages (e.g., to accommodate larger wafers), higher velocities, and greater acceleration (e.g., 2 g).
The control force in a maglev stage is provided by actuators magnetically coupled to a mass (e.g., a stage base) significantly more massive than the platen. The application of force to the platen results in a reaction force that is transmitted to the larger mass through a structural support. This reaction force can create undesirable broadband vibrations that can be transmitted to other portions of the lithography system, as well as to tools and instruments residing nearby. In the next-generation stages, the broadband vibrations from these reaction forces will be significant and have the potential to adversely affect the performance of the lithography system.
Typical prior art maglev stages have a wafer carrier that has a first section kinematically mounted to a larger second section. This mounted section supports the wafer and includes mirrors used by one or more interferometers to determine the stage position. A kinematic mount is used to decouple the interferometer mirror from control forces that could distort the mirror surface. The need for a kinematically mounted wafer carrier section in prior art maglev stages is due to the fact that the applied forces that position the stage are overconstraining. An overconstralning force is one that when applied will not change the stage position or orientation but will instead distort the structure. Unfortunately, the need for a kinematically mounted section complicates the design of the stage.
Thus, as the demands on the lithography process continue to grow, so do the demands on stage performance. Accordingly, improved maglev stage designs are needed to meet the heightened lithography process demands.
A first aspect of the invention is a magnetic levitation (maglev) stage apparatus. The apparatus includes a platen having an upper surface capable of supporting a workpiece. In an example embodiment, the platen is formed as a grid made up of carbon-fiber sheets. A set of magnet arrays is arranged on the bottom surface of the platen. First and second side magnet arrays are arranged on opposite sides of the platen. A support frame at least partially surrounds the platen on the opposite sides and the bottom surface. Motor coils are arranged on the support frame so as to be in operable communication with the set of magnet arrays and the side magnet arrays. The magnet arrays and motor coils are operable to magnetically levitate the platen within the support frame and move it in up to six degrees of freedom (DOF). The center of force applied to the platen is coaxial with the stage principal axes in the three translational DOF (i.e., X, Y and Z).
A second aspect of the invention is the above-described stage apparatus, further including movable counterweights arranged adjacent the outer sides of the support frame. The counterweights are adapted to move in opposition to the platen to cancel the platen accelerating forces.
A third aspect of the invention is a method of moving a workpiece supported by a platen. The method includes arranging a plurality of magnet arrays on one or more of the platen surfaces so that the magnet arrays are arranged symmetrically about the center of gravity of the platen. Further, motor coils are provided on a support frame that partially surrounds the platen. The motor coils are then operatively coupled one to each magnet array so that one or more forces may be applied to the platen along one or more axes passing through the center of gravity of the platen to move the platen in up to six degrees of freedom.
A fourth aspect of the invention is the above-described method, wherein the counterweights are moved in opposition to the motion of the platen to cancel the platen accelerating forces.